How It WOrks

Five steps to fresh, clean drinking water

Click to download a larger version.

1. PretreatmentPretreatment is the first stage of the desalination process. When seawater arrives at the plant, it goes through a pretreatment process to remove algae, organic materials and other particles. Seawater is pumped into multimedia filter tanks, which include layers of anthracite and sand atop a bed of gravel. Once filtered, the water moves into the next stage of desalination.

​2. Secondary PretreatmentBefore seawater enters the reverse osmosis filters to remove the salt particles, it must go through a second stage of pretreatment called microfiltration to remove smaller – oftentimes microscopic – impurities. At this point, virtually all impurities other than dissolved salts and minerals have been removed from the water, but it still needs to go through one more step to remove the dissolved salts and minerals to be ready for drinking.

Click to download a larger version.

3. Reverse Osmosis Building – The “Heart” of the PlantThe reverse osmosis building is the center of the desalination process, and the Carlsbad Desalination Plant. During this process, dissolved salt and other minerals are separated from the water, making it fit for consumption. The reverse osmosis building contains more than 2,000 pressure vessels housing more than 16,000 reverse osmosis membranes. The pressure vessels were provided by San Diego County-based Protec Arisawa and Dow Water & Process Solutions produced the reverse osmosis membranes.Reverse osmosis works by pushing water – under intense pressure – though semi-permeable membranes to remove dissolved salts and other impurities. These membranes act like microscopic strainers that allow only water molecules to pass through, leaving behind the salt, minerals and other impurities such as bacteria and viruses. In addition to the reverse osmosis membranes and pressure vessels, this building houses 144 state-of-the-art energy recovery devices produced by Energy Recovery, Inc. The energy recovery devices work by capturing the hydraulic energy created by the high pressure reject stream of seawater produced during the reverse osmosis processes and transfers it into incoming seawater, without consuming any electrical power themselves.These devices save the plant an estimated 146 million kilowatt-hours of energy per year, reducing carbon emissions by 42,000 metric tons annually – roughly equivalent to the annual greenhouse gas emissions from 9,000 passenger vehicles.

Click to download a larger version.

4. Post TreatmentAfter reverse osmosis filtration, the fresh water is nearly ready for consumption. But before making its way to your faucet, the water must undergo “post treatment.” This includes adding some minerals back into the water and disinfection with chlorine.

​5. Product Water StorageOnce the desalination process is complete, the water moves to the product water tanks, where it is then pumped 10 miles to the San Diego County Water Authority’s Second Aqueduct in San Marcos. Here, the water is blended with the regional supply and transported to a faucet near you.

​To Learn More

For further information about reverse osmosis, desalination and Poseidon, please visit the following websites: